A biological model to test the neurotoxicity of water soluble contrast agents was developed in this laboratory. It involves the intrathecl administration of water soluble contrast agents in cats and the study of: 1. The evoked cortico-spinal responses. 2. Evoked segmental responses. 3. The use of modifying drugs to understand the mode of action of the various contrast agents on the central nervous system. Valium and other suppressing agents are being investigated. The data to date indcat that there is a significant decrease in te evoked corticospinal responses using meglumine diatrizoate and the lesser increase with meglumine iothalamate. The chanes with amipaque are less obvious.